A color Doppler method in ultrasonic diagnosis is configured to transmit an ultrasonic wave to a living body in the same direction a plurality of number of times. This method extracts blood flow information such as the velocities, variances, and powers of blood flows from a plurality of echoes using the Doppler effect. Echo data strings, of the data obtained by plural emissions of ultrasonic waves in the same direction, which are associated with the same point (the same depth point on the same ultrasonic raster) are called packets. The packet size is set to about 5 to 16, that is, packets are made uniform by emitting ultrasonic waves 5 to 16 times in the same direction. A blood flow signal is extracted from a packet by applying to it a wall filter for suppressing signals from the tissue, i.e., clutter signals, thereby displaying blood flow information such as a velocity, variance, and power. For this reason, the following problem arises.
A packet is closed within an ultrasonic scan frame. Therefore, as the packet size increases, the framer rate decreases. An IIR filter is often used as a wall filter. With a small packet size, however, a transient response occurs in the IIR filter, resulting in a deterioration in the characteristics of the IIR filter.
Studies have therefore been made on a method of handling signals at the same position between frames as a packet instead of handling a packet as a closed packed in a frame. In this method, since a pulse repetition frequency (PRF) is equal to a frame rate, the aliasing velocity becomes low to allow even low flow rate observation. However, the method makes it easy for clutter signals from the tissue to pass through a wall filter, and hence motion artifacts tend to occur. When, in particular, the operator moves the probe gripped by his/her hand, an entire window is displayed with clutter noise. This problem arises not only in the above scanning method but also in general color Doppler scanning operation when the aliasing velocity is low.